This invention relates to a radiation detection device, and more specifically to a radiation detection tube having a conductive external surface for minimizing spurious detection of secondary radiation. The invention is related to radiation sensitive, gaseous discharge detectors of the Geiger-Mueller type, capable of operating at low voltages and having high current carrying capacity.
A radiation detector of the Geiger-Mueller type has an anode and a cathode placed in a medium comprising an ionizable gas, and which, upon being subjected to radiation to which it is sensitive, causes electrons to be ejected from the cathode, which electrons are then accelerated toward the anode. The accelerating electrons collide with gas atoms, thereby ionizing the gas and releasing further electrons, which accommodate the flow of current between the anode-cathode electrodes. The current is subsequently monitored and controlled via external circuitry. One of many such control schemes is described here. When the current reaches a specified level the external circuits electrically short the anode to cathode which allows the ionized gas to return to the neutral state. A voltage is then reapplied to the anode, enabling the detector to further respond to radiation that may be present. The number of such gas discharge events per unit of time provides a measure of the amount of radiation incident upon the tube, and external circuits are utilized to count and otherwise record these discharge events.
One such radiation detector of the Geiger-Mueller type is described in U.S. Pat. No. 2,461,254, issued Feb. 8, 1949. Another patent in the prior art which discloses a similar Geiger-Mueller type detector is U.S. Pat. No. 2,500,941, issued Mar. 21, 1950. A further patent disclosing a similar invention is U.S. Pat. No. 3,344,302, issued Sept. 26, 1967. These patents generally disclose anode-cathode relationships, enclosed in a gaseous medium, with a glass envelope surrounding the components and medium, so as to provide electrical signals representative of radiation incident upon the device.
There has always been a problem with these and other prior art devices with respect to the detection of spurious secondary radiation. Spurious secondary radiation is defined as radiation that is capable of producing a discharge in the previously mentioned devices which is initiated by radiation from sources other than sources that the device was designed to respond to; i.e., the ultraviolet component of solar radiation. Attempts have long been made at minimizing this spurious secondary radiation, and it has been thought that the solution to the problem resides in the proper design of the anode/cathode configuration and the electrode to glass spacing, together with the proper selection of the electrode materials and fill gases used for these components. It was felt that spurious secondary radiation could be controlled, or at least minimized, by proper selection of these design parameters, and previous attempts at minimizing spurious secondary radiation have been directed in these areas.
The present invention results from the discovery that the spectral response is derived not only from the configuration described above, but also from the glass envelope of the device to an extent not previously known. Large spacing of the electrodes from the internal walls of the device had been thought to be an adequate measure to eliminate the influence of the glass surfaces. Upon discovery that this is not reliable under all operating conditions of the device, and that it is influenced by the electrostatic history of the tube, the factors which might influence or limit the extent of the glass spectral response were investigated. It is believed that the spectral response of the glass is due to the photoelectron emission of the glass from what is known as "electron traps" or "acceptor states" in the glass. The acceptor states are believed to be intrinsic in the glass, or they may be due to the various process steps involved in manufacturing the tube, or both.
Williams has described an experiment wherein light illumination of SiO.sub.2 layers caused the release of electrons trapped in the SiO.sub.2, in a study he made of the transporting and trapping properties of electrons in SiO.sub.2 layers. This experiment is summarized in a paper entitled "Internal Photoemission as a Tool for the Study of Insulators," by Alvin M. Goodman. The paper is found at page 99 of Optical Properties of Dielectric Films, published by the Dielectrics and Insulation Division of the Electrochemical Society, Inc., New York, N.Y. (1968). The Goodman paper discusses this process, which it refers to as "internal photoemission;" i.e. the photoemission of mobile electrons or holes into an insulator from an adjacent conducting medium. Goodman provides six different examples of the internal photoemission process, and it is believed that at least some of these examples contribute to the understanding of how acceptor states in glass may be populated with electrons. Acceptor states can be populated with electrons naturally, or they can be populated by the abundance of electrons available during the operation of the tube. Once these acceptor states are populated, it is then possible to have photoelectron emission from the glass, and it is believed that this photoelectron emission is a principal cause of the spurious secondary radiation commonly detected by such devices.
The present invention is primarily concerned with radiation detectors responding to ultraviolet radiation whose wavelength is below the ultraviolet component of solar radiation. It is believed that radiation in this range, as well as radiation whose wavelength is in the ultraviolet component of solar radiation begins the process of photoelectron emission when such radiation passes through the glass. When electrons are produced from the glass, according to this mechanism, they may be injected into the interior of the tube, and may accelerate toward the anode and cause spurious counts.
When a voltage potential is applied to the glass envelope it has been found to have a direct effect on the secondary photoelectron emission process. Applying a voltage potential which is negative in the same sense as the cathode voltage has been found to increase the amount of spurious secondary radiation present and detected by the tube. Applying a zero voltage potential, or grounding the glass envelope, has also been found to increase the spurious radiation detected. Further, it has been found that applying a fixed positive voltage potential to the glass envelope produces different but equally unacceptable results. Experimentation has shown that the best results are achieved when the voltage potential applied to the glass envelope is made to follow the anode voltage, which varies during operation of the detector, and the present invention relates to an apparatus for accomplishing this result.